1. Field of the Invention
This application relates generally to hydraulic valves, and in particular to valves for controlling hydraulic actuators, for example, actuators associated with pump/motors.
2. Description of the Related Art
FIG. 1 shows a hydraulic actuator 100, including a piston 104, a cylinder 102, and a shaft 110. The piston 104 has a surface 104a that, in use, is subject to fluid pressure. Surface 104a may be referred to herein as the open side, working side, head side, or large side. The piston 104 also has a surface 104b, referred to herein as the shaft side, due to the presence of the shaft 110 coupled thereto. Other terms used in the art include piston rod side, annular chamber side, and small side. It will be understood that the selection of terms is irrelevant to the function of the device, and has no bearing on the scope of the invention or claims.
Such an actuator is operated by providing pressurized fluid at port 114 to a shaft side chamber 108, and selectively providing pressurized fluid at port 112 to an open side chamber 106. If fluid force against the open side surface of the piston 104 exceeds a force against the shaft side surface of the piston, the piston will rise, as viewed in FIGS. 1-3. Conversely, if the force exerted by pressurized fluid against the shaft side surface 104b of the piston 104 exceeds the force of fluid against the open side surface 104a, the piston 104 will drop. The position 104 of the actuator 100 is controlled by controlling the fluid pressure in the open side chamber 106 of the cylinder 102 of the actuator 100. However, it will be noted that the surface area of the shaft side surface 104b of the piston 104 is less than that of the open side surface 104a of the piston 104, owing to the volume of the shaft 110, which reduces the surface area of surface 104b. Accordingly, an equal fluid pressure in each of the shaft side and open side chambers 108, 106 of the cylinder 102 will result in a greater force being exerted on the open side surface 104a of the piston 104 than on the shaft side surface 104b. Thus, if the fluid pressure in the shaft side and open side chambers 108, 106 of the cylinder 102 is equal, the piston 104 will rise.
Control of such an actuator may be achieved through the use of an actuator control valve such as that shown at reference numeral 116. The actuator control valve 116 is controlled by a solenoid 132, which is in turn controlled by an electronic control unit voltage command signal 154. The force exerted by the shaft 134 of the solenoid 132 on the spool 118 of the valve 116 is determined by the voltage level provide by the command signal 154. The force exerted by the shaft 134 on the spool, in opposition to a biasing force of the spring 138, controls the position of the spool 118 within the valve housing 117. The valve 116 includes three ports, 126, 122, 124. The first port 126 is coupled to a high-pressure fluid source 150. The third port 124 is coupled to a low-pressure fluid source 152, while the second port 122 is coupled to the open side port 112 of the actuator cylinder 102 via control line 128.
It should be noted that the shaft side port 114 of the actuator is coupled directly to the high-pressure fluid source 150 via high-pressure supply line 130. The spool 118 includes an annular channel 120, which is configured to link either the high-pressure fluid source 150 or the low-pressure fluid source 152 to the second valve port 122 and to the open side port 112 of the actuator 100. The spring 138 biases the spool 118 in an upward direction. Thus, when the solenoid 132 is activated to press downward on the spool 118, the spring 138 is compressed as the spool 118 drops.
Actuators of the type described above are sometimes referred to as differential actuators, because they respond to a difference in force against the respective surfaces of the piston. The relative forward and reverse response of such an actuator can be selected by selecting the area of the shaft and the pressure applied to the open side chamber 106. For example, assuming the cylinder 102 has a transverse sectional area of two square inches, and the shaft 110 has a transverse sectional area of one square inch, the effective surface area of the shaft side surface 104b of the piston 104 will be one square inch, while the effective surface area of the open side surface 104a of the piston 104 will be two square inches. Further, assuming a high-pressure source 150 of 1,000 psi, and a low-pressure source 152 of 20 psi, coupling the high-pressure source 150 to the open side chamber 106 means that the force acting on the open side surface 104a of the piston 104 is:
                              1          ⁢                      ,                    ⁢          000          ⁢                                          ⁢          pounds                          in          2                    ×      2      ⁢                          ⁢              in        2              =          2      ⁢              ,            ⁢      000      ⁢                          ⁢      pounds        ,
While the same high pressure in the shaft side chamber 108 results in a force acting on the shaft side surface 104b of the piston 104 of:
                    1        ⁢                  ,                ⁢        000        ⁢                                  ⁢        pounds                    in        2              ×    1    ⁢                  ⁢          in      2        =      1    ⁢          ,        ⁢    000    ⁢                  ⁢          pounds      .      
The differential force, then, is 2,000 pounds−1,000 pounds=1,000 pounds, pushing the actuator 100 toward the shaft side. On the other hand, if the low pressure 152 is applied to the open side chamber 106, the force acting on the open side surface 104a of the piston 104 is:
                              20          ⁢                                          ⁢          pounds                          in          2                    ×      2      ⁢                          ⁢              in        2              =          40      ⁢                          ⁢      pounds        ,while the force acting on the shaft side surface of the piston remains at 1,000 pounds. Accordingly, the differential force is 1,000 pounds−40 pounds=960 pounds, pushing the actuator 100 toward the open side of the piston 104.
It will be recognized that, by selecting the diameter of the shaft, relative to the diameter of the cylinder, the forces acting on the actuator in a forward direction and a reverse direction may be made to be approximately equal, as described above, or may be made to operate with much higher forces in one direction than the other. It will also be recognized that the relative pressures of the high and low pressure fluid supplies, and the dimensions of the actuator, may be selectively modified according to the particular application, with the values used above being selected for purposes of illustration only.
FIG. 1 shows the actuator valve 116 with the spool 118 in a first, upper position. In this position, the annular channel 120 is positioned to couple the high fluid pressure at the first port 126 with the open side chamber 106 of the actuator 100, via the second actuator control valve port 122 and the pressure line 128. Accordingly, fluid from the high-pressure fluid source 150 is driven into the open side chamber 106 of the actuator 100. As previously explained, even though the shaft side chamber 108 of the actuator 100 is coupled directly to the high-pressure fluid source 150, an equal pressure in the open side chamber 106 of the actuator 100 is sufficient to drive the piston 104 of the actuator 100 upward. Accordingly, when the spool 118 is in the first position, as shown in FIG. 1, the piston 104 of the actuator 100 is driven upward.
FIG. 3 shows the actuator control valve 116 with the spool 118 in a third, lower position. In this position, the annular channel 120 couples the low-pressure fluid source 152, to the open side chamber 106 of the actuator 100, via the second valve port 122 and the pressure line 128. In this position, the high pressure in the shaft side chamber 108 of the actuator 100 is sufficient to drive the piston downward against the low pressure, in the open side chamber 106 of the actuator 100.
It will be noted that there is a linking arm 136, which serves to couple the actuator shaft 110 to the spring 138. The linking arm 136 provides positional feedback to the actuator valve. As the actuator shaft 110 drops, the linking arm 136 compresses the spring 138. When the increasing upward force exerted by the compressed spring 138 exceeds the downward force exerted by the solenoid 132, the spool valve 118 will be pressed upward into the second position, as shown in FIG. 2. This may occur at any point in the travel of the piston 104, in as much as the force exerted by the solenoid 132 is variable, based upon the voltage supplied by the command signal 154.
FIG. 2 shows the spool 118 in a second, central position. As may be seen, the annular channel 120 is not in fluid communication with either the first port 126 or the third port 124. Thus, the second port 122 is coupled to neither the high-pressure fluid source 150 nor the low-pressure fluid source 152. In this position, the actuator control valve 116 arrests the piston 104 at any desired position. Because the fluid in the pressure line 128 and the lower chamber 106 is incompressible, the high-pressure fluid of the upper chamber 108 cannot drive the piston 104 downward.
Finally, when the spool 118 is in the first position, causing the actuator shaft 110 to rise, as previously described, it may be seen that the linking arm 136 progressively reduces the upward bias on the feedback spring 138 as the shaft 110 rises. If, during the upward travel of the actuator, the upward biasing force applied by the spring 138 on the spool 118 drops below the downward biasing force applied by the shaft 134 of the solenoid 132, the spool 118 will drop into the second position, decoupling the open side chamber 106 from the high pressure fluid source 150, and arresting the piston at that position.
Various valve configurations and systems for controlling actuators are described in the following patents, which are incorporated herein by reference in their entireties: U.S. Pat. No. 4,311,083, issued to Guillon; U.S. Pat. No. 4,958,495, issued to Yamaguchi; and U.S. Pat. No. 5,421,294, issued to Ruoff, et al.